1. Field of the Invention
The present invention relates to a method for fabricating a thin layer device, and in more detail the present invention relates to a method for fabricating a thin layer device such as a superconductive device having an SIS junction on a SiO2 substrate, and a superconductive device.
2. Description of the Related Art
In the fields of global environment measurement or radio astronomy and the next generation wireless communication, development of a receiver or oscillator for a submillimeter band has been desired, and research and development of a superconductive device such as the Josephson junction has been made as an example. A superconductive SIS (Superconductor-Insulator-Superconductor) junction which is one of the Josephson junction flows superconductive tunneling current between two superconductive electrodes up to a critical current, while generation of voltage between two superconductive electrodes over this value. Moreover, current-voltage characteristics of the junction have a very strong nonlinear characteristics not attained by a semiconductor device.
At present, in a frequency range between 300 GHz to 725 GHz, an SIS receiver using the SIS junction shows the lowest noise temperature, for example, an Nb-based SIS receiver used for up to 725 GHz, which is the gap-frequency of niobium (Nb), having super low noise has already been developed. However, in frequency range over 725 GHz, performance of an SIS receiver is abruptly deteriorated due to increase in electrode loss caused by superconductive electron-pair breaking in Nb electrodes.
Under these circumstances, development of an SIS receiver using niobium nitride (NbN) having large superconductive gap energy and low loss characteristics up to 1.4 THz, has been carrying out. However, this low loss characteristics of NbN largely depends on crystallinity of an NbN thin film, and therefore introduction to development of a submillimeter band device of NbN requires device production technology while maintaining good crystallinity. In other words, NbN-based SIS junction is required to be composed of only a multilayer film of epitaxially grown NbN, MgO, and the like.
On the other hand, SiO2 having low dielectric constant has conventionally been used as a substrate for producing a submillimeter band receiver, however, there are no reports on epitaxial growth of NbN in the case of using a SiO2 substrate, and a rock salt-type single crystal substrate such as MgO is widely used at present. Practically, formation technology of an SIS junction composed of an NbN layer/an MgO layer/an NbN layer (hereinafter called as an NbN/MgO/NbN-SIS junction) on an MgO substrate has been disclosed (JP-A-2001-352109). In the publication, an NbN layer, a thin MgO layer and further an NbN layer are each formed by epitaxial growth on an MgO substrate.
In this connection, a waveguide mixer for a submillimeter band is required to be produced on sufficiently thinner substrate than wavelength in the waveguide. As a guideline of substrate thickness of a mixer for 1 THz waveguide, not thicker than ¼ wavelength of electromagnetic wave in a substrate, namely, not thicker than about 40 μm for a SiO2 substrate and not thicker than 27 μm for an MgO substrate is required. Production of a device using a substrate having this level of thickness while maintaining sufficient surface area of the substrate is difficult, and therefore, generally, an SIS junction is produced on a substrate having a thickness of several hundreds μm, and then the substrate is subjected to mechanical polishing to finally adjust to objective substrate thickness.
An NbN-based SIS mixer produced on an MgO substrate by such a method exhibits certain level of performance, when introduced to a mixer for a submillimeter band waveguide (Waveguide-type all-NbN SIS mixers on MgO substrates by Masanori Takeda et al., Advanced Research Center, National Institute of Information and Communications Technology).